Electromagnetic forming is a method in which a momentary large current is applied to a coil to generate a strong magnetic field such that an object (conductor) disposed in the magnetic field is molded by an interaction between an eddy current generated at the object and the magnetic field. The electromagnetic forming is a known technique disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 6-312226, 9-166111, 58-4601, and the like. The momentary large current is a current with a value of, for example, 10 kA or more.
Japanese Unexamined Patent Application Publication No. 6-312226 discloses an application of this electromagnetic forming to processing of a cylindrical member (hollow workpiece). According to the description, a coil for electromagnetic forming is inserted in the hollow workpiece of an extruded aluminum material, and molds for forming an expanded-tube shape are disposed adjacent to the outer circumference of the hollow workpiece E having a length corresponding to the coil. By applying a momentary large current to the coil in this arrangement, the outer circumference of the hollow workpiece is pressed toward the molds, and thus the outer circumference is molded into a shape corresponding to the molds. In this manner, a hollow component of which the cross-section is changed in the longitudinal direction is produced.
However, when the above-described method was simply applied for manufacture of a cylindrical ring with beads, a dimensional accuracy of products was insufficient, and the productivity could not be increased either. In order to put the method for manufacturing the cylindrical ring with the beads by electromagnetic forming to practical use, various additional schemes have been required.
The present invention is produced so as to solve the problems of the known method for manufacturing the cylindrical ring with the beads. It is an object of the present invention to produce highly accurate cylindrical rings with beads at low cost and with high productivity.